The invention relates to a method and apparatus for controlling coordinate displacements of a platform. The preferred application of the invention is a microscope stage.
In general microscopy, the subject of the investigation is supported on a stage. The subject may be a slide, i.e. a package of two glass plates enclosing a specimen in the form of a layer. The slide is supported by the stage which functions like a platform and defines the object plane of the microscope. The slide plane is referenced in conventional manner having coordinates x and y.
If the specimen or probe to be viewed has an area larger than the area of the microscopic field of view, as is normally the case, the user has to select a specific area which is just under observation. The slide may be dislocated by freely shifting it on a flat surface of the platform. Otherwise the field selection is accomplished by displacements of the stage in the x,y-plane. In other words, the platform carrying the fixed specimen slide may be shifted along the x-axis and the y-axis independently.
In pathology, the microscope is the basic instrument used to visualize specimen. The microscopic analysis satisfies diagnostic purposes of various kinds. In the past decade many attempts have been made to improve the reliability of the microscopic analysis. In addition to the qualitative information which is traditionally collected by a microscope, more and more quantitative information is gained. In order to collect the quantitative information, a computer-based image analysis system has been established around the microscope. Most of the known image analysis systems are of an interactive nature, i.e. the user performs a dialogue with the programmed computer in the course of collecting the optical information. An important interactive contribution of the user is to select the microscopic field to be measured.
Normally it is intended in such interactive image analysis systems to screen the complete slide only once. There is a need for an economical, systematic investigation of specimen having a relatively large area as compared to the necessary resolution of details. The machine performance may be enhanced by tracing and recording the x-y-position of the stage. Thus, the computer may not only forward position displacement signals to the microscope, but also gets a feed back signal on the position of areas which have already been screened. This means that the microscopes stage should be equipped with position read-out facilities.
There are two principal ways to perform the field selection when the slide is fixed on the platform: 1) the classical mechanical drive by two direct handling wheels or 2) an electro-mechanical drive which is typically used in conjunction with x,y-controllers of computers. Both principles will be explained now by referring to the prior art.
European Patent Application EP 317139 discloses a system for measuring selected features of cell parameters by optical identification of marked cells. The known system functionally operates as a digital image analysis and processing system. The apparatus comprises a high resolution microscope enabling an operator to view magnified specimen on a support, in a preferred embodiment a glass slide. The microscope includes adjustment or positioning means for focusing its optics on the slide and a platform movable incrementally in two directions via positioning means in order to view different areas thereof. Positioning means are in the form of mechanical adjustment verniers which are conventional for instrument quality microscopes.
The known microscope stage is fitted with an x-y sensing device having an accuracy of about 10 .mu.m in the microscope slide plane. These sensors are computer-interfaced and are constantly read by the system. They are used with special microscope slides having marks on the glass plates to record the relative x-y position of any image acquired by the system. This x-y-position detecting and recording mechanism is useful in experimental situations where a great amount of image information has to be evaluated. The microscope slide itself may be used, so to say, as an image storage medium. The microscope slide is accessed just like an image storage medium, e.g. when experiments are repeated or when a back-up memory for the digital image store is helpful or when complete sets of digitized images would require an enormous storage capacity.
A second example of a known microscope is marketed under the tradename Westerboer. The stage of Westerboer has--for each direction x, y--two different parallel-acting drive means in order to control the displacements in the x,y-plane. The first drive means is the conventional mechanical control of a microscope stage. The second drive means is a computer-aided electro-mechanical control of the stage. Both the mechanical and the electro-mechanical drive are linked in parallel to the known stage.
The conventional manual drive of the Westerboer microscope stage is operated by means of two cascaded wheels placed on the same axis. With the aid of gears the wheel movement is translated into the stage movement.
The second drive means of the known stage employs a so-called mouse or a so-called joy-stick to input desired x and y coordinates to a computer. The position command signals are decoded in the computer and transformed into corresponding drive signals. The drive signals are outputted from the computer to control a x-motor and a y-motor. The x-motor rotates a spindle having a threading for s a correspondingly threaded member of the stage. The y-motor rotates a y-spindle; the y-spindle rotation is transformed into a shift of the stage in y-direction by another threaded member. The motors for driving the known stage are DC servo motors which act in the known manner by comparing an input DC signal and a DC feedback signal of a position sensor belonging to the servo motor. If the difference between both DC signals is zero, the DC motor stops at the desired position.
The spindles of the electro-mechanical drive and the racks of the mechanical drive are commonly attached to the stage. The user decides whether he wants to actuate the conventional manual drive or the joystick/mouse of the computer-aided drive. Both drive means are mechanically interconnected by the driven stage itself. On actuation of the x-handling wheel, the displacement of the stage leads also to a rotation of the x-motor spindle. On the other hand, by actuating the mouse or the joy-stick, the DC-motor will not only shift the stage, but will also move the drive mechanism of the cascaded wheels. Hence, a mechanical connection exists between the handling wheels and the servo motors.